Introduction to Computation for the Humanities and Social Sciences. CS 3 Chris Tanner

Transcription

1 Introduction to Computation for the Humanities and Social Sciences CS 3 Chris Tanner

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3 Lecture 5 But I ve got a blank space [in my text String] baby Taylor Swift

4 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!4

5 Reading Input Programs need data/input in order to operate Input can come from: someone using your program external source (e.g., files on your computer, live web page like Twitter, etc)!5

6 Reading Input from a User 1 2 name = input( What is your name? ) age_string = input( What is your age? ) A function that collects input from the command-line terminal, and returns (aka outputs) the input value as a text string Useful when used in a program (.py), not in Python s interactive mode The input argument prompt will be displayed to the user on the same line as their input, so it s helpful to include some whitespace at the end of your prompt so the user can easily see the start of their input!6

7 Reading Input from a User Everything returned by input() will be a text string, so if you re actually wanting text data, that s fine. Otherwise, if you want to store the text input as a numerical data type, we need to cast it accordingly. 1 2 age_string = input( How old are you? ) age = int(age_string)!7

8 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!8

9 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!9

10 Reading Input Computer Storage VS Large, slow, permanent Small, fast, temporary!10

11 Reading Input The input() function is fairly impractical 1 2 script_text = input( Type in the script for \ The Office - Season 5, Episode 7: )!11

12 Reading Input from a File Opening/Closing file With our programs so far, we have been storing data to variables. This occurs in our computer s memory (which is temporary) The computer does not remember this information after your program is run Your computer can also permanently store information on its hard drive Saving your program s file (.py code) is an example of permanently writing your file to the hard drive, as is any other document you write Reading and writing files from/to the hard drive represents another input/output method for our programs!12

13 Reading Input from a File Opening/Closing file For both reading and writing, you need to write Python code to open the file After you are done reading/writing, your program needs to close it When writing to a file, if you don t close your file, your computer will explode the changes you made might not be saved After closing the file, you can t read or write from it again without opening it again!13

14 Reading Input from a File Example 1 input_file = open( example.txt, r )!14

15 Reading Input from a File Example 1 input_file = open( example.txt, r ) the filename of the actual file that already exists on your computer, which we ll read. specifies to open the file for reading purposes.!15

16 Reading Input from a File Example 1 input_file = open( example.txt, r ) the contents of the file are now temporarily stored/accessible via this variable we re creating!16

17 Reading Input from a File Example 1 input_file = open( example.txt, r )!17

18 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: print(line) input_file.close()!18

19 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: print(line) input_file.close() must be 4 spaces!19

20 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: print(line) input_file.close() NOTE: the end of every line you read will contain a special character (i.e., pressing ENTER) which represents end of line, go to the next line. To remove this extra spacing, we need to call the function.strip()!20

21 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: line = line.strip() print(line) input_file.close()!21

22 Reading Input from a File Example It s practice to run.strip() on every line from an input file If the file is delimited by a certain character, we can split() the line up into each of its meaningful pieces (i.e., columns of data) some files are delimited by a comma (.csv files), which denotes each piece of data from the other other times, just separating a line by its spaces could be highly useful e.g., just loading every word from a line!!22

23 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: name, age, weight = line.strip().split(, ) print(weight) input_file.close() Michael Scott,47,165 Jim Halpert,33,178 Dwight Shrute,36,183 input: example.txt!23

24 Reading Input from a File Example input_file = open( example.txt, r ) for line in input_file: name, int(age), int(weight) = line.strip().split(, ) print(weight) input_file.close() Michael Scott,47,165 Jim Halpert,33,178 Dwight Shrute,36,183 example.txt!24

25 Reading Input from a File Filename vs File input_file = open( example.txt, r ) for line in input_file: line = line.strip() file variable filename print(line) input_file.close() The filename is just a string that represents the path to the file from the location you ran your program from The file variable connects the program to the file on the computer and its contents!25

26 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!26

27 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!27

28 Writing to a File Opening a File to Read: 1 input_file = open( example.txt, r ) Opening a File to Write: specifies it s for writing 1 output_file = open( example.txt, w )!28

29 Writing to a File Opening a File to Read: 1 input_file = open( example.txt, r ) Opening a File to Write: output_file = open( example.txt, w ) output_file.write( Hello! ) output_file.close()!29

30 Writing to a File Opening a File to Read: 1 input_file = open( example.txt, r ) Opening a File to Write: my_name = Chris output_file = open( example.txt, w ) output_file.write( Hello, my name is: + my_name) output_file.close()!30

31 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!31

32 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!32

33 Errors We ve seen three specific errors so far (in Lecture 4): NameError, TypeError, and ValueError In general, there are three categories of errors: syntax errors runtime errors logic errors (includes TypeError) (includes NameError and ValueError)!33

34 Errors Syntax Errors If your code is improperly formed, a syntax error will occur during the compilation stage (before your code is executed, and when your code is being translated to lower-level byte-code). They show up immediately, no matter where the issue exists in the code These are basically grammar issues, where you re doing something that Python is smart enough to know ahead of time is wonky. The easiest of the errors to fix.!34

35 Errors Runtime Errors A runtime error occurs when your code is actually being executed, and Python can t perform what you instructed it to do. Very common. Anytime you re trying to access something incorrectly according to how you ve defined your variables/data. Examples include mistyped variable names, using a variable before it s defined, etc.!35

36 Errors Runtime Errors An exception is thrown whenever the interpreter reaches a line of code that results in an improper calculation These exceptions occur at the specific point in time the improper calculation is encountered For a long running program, these may occur late in your program!36

37 Errors Logic Errors Logic Errors is a general term used to describe when your code executes but gives different behavior/results than what you intended. May or may not yield a runtime error. It s easy to define computations incorrectly, so start small, check your intermediate values often, and be methodical and patient (per Lecture 2)!37

38 Errors Error Handling If you anticipate that some data may differ from how you intend, you can write code to appropriately/gracefully respond, instead of just crashing. Use Try to represent the main action that ideally want to execute Use Except to anticipate a potential error type that would occur if used improperly Within the Exception, do whatever you d like in terms of recovering/ fixing the error or just informing the user and optionally exiting the program.!38

39 Errors Type Checking input_file = open( example.txt, r ) for line in input_file: name, age, weight = line.strip().split(, ) try: weight = int(weight) except TypeError: print( ERROR: Please enter an integer \ (whole number) for weight! ) exit(1) print(name + weighs + str(weight) + pounds ) input_file.close()!39

40 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!40

41 Lecture 5 Reading Input From Users Reading Input From Files Writing Data to Files Errors Style Guide!41

42 Style Guide Having correct code is the goal and most important element of programming However, the exact words you type (e.g., names you give to variables) also have some rules to follow some of which are mandatory by Python, others are just generally good practices for ensuring readable, nice-looking code.!42

43 Style Guide Python s Naming Rules for Variables Variable names must contain only the characters a-z, A-Z, 0-9 and underscores A letter or underscore must be the first character Variable names are case sensitive weight_per_pound result help! don t have a cow result_2 boys_2_men 2nd_result 98_degrees result is not same as Result!43

44 Style Guide Variable Names Allude to Type ints: num_employees, paper_count, list_index, etc floats: temperature, employee_weight, miles_travelled, etc strings: filename, book_title, word, sentence, book_description, employee_name, etc booleans: is_available, is_sorted, is_full, etc!44

45 Style Guide Python s Style Rules If a line of code extends beyond 80 characters long, break it up onto another line via wrapping parts in ( and ) or a \ at the end of the long line View -> Ruler if using Sublime) Example, some code of mine:!45

46 Style Guide Python s Style Rules Later, much of our code will need to be indented (as we saw for File reading/writing and lines that are > 80 characters) Always indent by typing 4 spaces (not TAB). Or, if you use TABS, make sure you tell your IDE (e.g., Atom, Sublime) to convert TABS to SPACES)!46

47 Style Guide Python s Style Rules For a comprehensive set of the Python s Style Guide (which helps answers nuanced cases), please visit the PEP 8 Style Guide!47

48 Style Guide Comments very useful: code can be difficult to read, not only for others but even to yourself later on to make it more readable, we can write some typical English comments/notes in our code, wherever we want! single-lined comments must be prefaced with # # tells Python to ignore that line, as it s not Python code if you have several lines of comments, you can just surround the text with before and after!48

49 Style Guide Comments Examples: # initializes variables my_age = 18 my_name = Charlie looks at votes from all senators over the past 10 years, and for each year displays the top 5 senators who voted most dissimilar from the rest of their political party def loadlistofsenators(file): code here!49

50 Style Guide Comments It s recommended to comment main sections of code Commenting too much is ridiculous and distracting Here s a healthy amount:!50

51 Style Guide Our Style Rules Only use lowercase characters Separate words with underscores Variable names should contain full words or common abbreviations (e.g., avg) Don t make up your own abbreviations or shorthand (e.g. s for sum/string) Don't use a reserved Python keyword (any word not in quotes that s colored in your text editor) Including a Python word as part of the variable name is OK (weight_string, user_input) Variable names should include a word that alludes to its type!51

52 Style Guide Name That Variable The number of senators in a file The average age of a group of people The name of the user of our program, per their input A date of birth for someone The name of a city about which we will analyze data The total number of people affected by a power outage!52

53 Lab Time